First Go to admin/backend Components -> jomclassifieds -> Global Configuration -> site display settings -> Show Price (Enable it) and then Go to "Price categories" below the show price field.

After enabling “Show price Before/After Currency” should select the categories for the price field to be active. Selected categories will have the price field and non-selected categories won’t have price field. Changes will affect in advert detailed/list view and post free advert form.